(1) Field of the Invention
The present invention relates to an apparatus for and a method of controlling calls, and a storage medium which stores a call control program, and more particularly to a call control apparatus for releasing a call, a call control apparatus for establishing a call, a call control method of releasing a call, a call control method of establishing a call, a storage medium which stores a call control program for releasing a call with a computer, and a storage medium which stores a call control program for establishing a call with a computer.
(2) Description of the Related Art
As the networking technology advances, needs for information communications increase, and services for providing such information communications are available in a diverse range. One service that is applicable to the NVOD (Near-Video-On-Demand) system employs a multicast call control function.
According to the multicast call control function, information such as sound and image information sent from a transmission terminal is copied in a switching exchange, and distributed to a plurality of reception terminals according to a point-to-multipoint multicast routing process.
A switching exchange with a multicast call control function establishes and releases calls between terminals and adjacent switching exchanges according to a user-network interface (UNI) or a network-network interface (NNI) which has been standardized by ITU-T.
A switching exchange with a multicast call control function copies information at a suitable branching point and sends the information to a plurality of users according to a point-to-multipoint multicast routing process.
FIG. 20 of the accompanying drawings shows in block form a connected configuration of a multicast call control system. In the multicast call control system, a transmission terminal for transmitting information is referred to as a root, and a reception terminal for receiving information is referred to as a leaf, with a switching exchange corresponding to a node.
In FIG. 20, a node N1 is connected to a root R1 and nodes N2, N3, N4 are connected to the node N1. Leaves L1, L2 are connected to the node N2. Leaves L3, L4 are connected to the node N3. Leaves L5, L6 are connected to the node N4.
Information transmitted from the root R1 is copied in each of the nodes N1-N4, and multicast to the leaves L1-L6.
To release the leaves L1-L6, an REL signal which represents a release request massage is issued from the root R1 for thereby releasing all the leaves L1-L6 which belong to the same call.
FIG. 21 of the accompanying drawings illustrates a releasing sequence for releasing the leaves L1-L6 of the multicast call control system shown in FIG. 20.
The releasing sequence shown in FIG. 21 is carried out as follows:
[S1] The root R1 transmits an REL signal which represents a release request massage to the node N1.
[S2] The node N1 transmits an RLC signal which is a response signal to the root R1.
[S3] The node N1 transmits the REL signal successively to the nodes N2, N3, N4.
[S4] The nodes N2, N3, N4 transmit respective RLC signals to the node N1.
The above releasing process is prescribed according to ITU-T recommendations Q.2971, Q2722.1. These ITU-T recommendations, however, fail to refer to the order in which a plurality of output paths issue respective release request messages. While the node N1 issues REL signals successively to the nodes N2, N3, N4 in the order named in FIG. 21, the node N1 does not need to issue REL signals in the illustrated order, but may issue REL signals in an arbitrary order.
The above releasing process does not take into account any efficient release order. Therefore, depending on the order of issuance of release request messages, a delay time spent after a root sends a release request to a network until a final leaf is released may become very long.
Another problem is that when a new multicast call is to be established after a root has issued a release request message to a network, if simultaneous releasing of a previously established multicast call is still uncompleted in the network, then designated terminals may possibly remain busy, and a new call may not be established to those designated terminals.